The present invention relates to a video signal processing system, and more particularly an automatic gain control circuit (AGC) thereof.
Generally an automatic gain control circuit controls input video signals at a given specified level in an image signal processing system such as a video tape recorder (VTR). Namely, it compares the amplitude of the synchronizing signal with a reference voltage to control the amplitude of the luminance signal at a reference value.
A conventional automatic gain control circuit, as shown in FIG. 1, comprises a first amplifier 1 with a gain varied according to a control voltage, a low-pass filter 2 for eliminating color signals, a sync separator 3 for separating synchronizing pulses, a delay circuit 4 for delaying the pulse from the sync separator 3, a clamper 5 for clamping the output of the low-pass filter 2, a first integrating circuit 6 for integrating the combined outputs from the delay circuit 4 and the clamper 5, a comparator 8 for comparing the output of the first integrating circuit 6 with a reference voltage adjusted by a variable resistor 7 to produce an error voltage, a second integrating circuit 9 for integrating the error voltage, and a second amplifier 10 for receiving the error voltage.
FIGS. 2A to 2F show the signal waveforms according to the automatic gain control circuit of FIG. 1, wherein FIG. 2A represents the input signal waveform to the automatic gain control circuit, FIG. 2B the output waveform of the low-pass filter 2, FIG. 2C the output waveform of the sync separator 3, FIG. 2D the output waveform of the delay circuit, FIG. 2E the output waveform of the clamper, and FIG. 2F the combined waveform of FIGS. D and E.
In this conventional automatic gain control circuit, the first amplifier 1 receives the video signals containing color signals of FIG. 2A. The low-pass filter 2 eliminates the undesired color video signals from the output signals of the first amplifier 1.
The sync separator 3 separates the synchronizing pulses from the output signal of the low-pass filter 2. And thereafter the output signal is delayed by the delay circuit 4 for a given time. The output signal in FIG. 2D is combined with the output signal in FIG. 2E of the clamper 5 in the mixer 11. The output signal in FIG. 2F of the mixer is transformed into a direct current voltage by the first integrating circuit 6, then compared with the reference voltage adjusted by the variable resistor 7 in the comparator 8. Thus the difference signal between the direct current voltage of the first integrating circuit 6 and the reference voltage is produced with a voltage gain. The output of the comparator 8 is received by the second integrating circuit 9 to produce an error voltage fed back via the second amplifier 10 to the first amplifier 1, thus controlling the voltage gain to obtain the desired video signals.
As stated above, this conventional circuit requires a variable resistor that should be manually adjusted in order to obtain the specified output value, making it difficult to automate a production line, thus increasing the production cost. Moreover the output waveform of FIG. 2F is integrated by the first integrating circuit 6, and therefore the output level of the circuit is changed according to the content of the video signals.